Belt alternator starter (BAS) systems for hybrid vehicles differ from conventional accessory drive belt systems in that the torque to turn the system is not always generated from the crankshaft. During normal operation the engine crankshaft provides torque for rotating a motor generator (MG) unit, or starter generator, generally providing taut and slack sides of the belt on opposite sides of the MG pulley. During engine starting, the MG unit drives the system causing the slack side of the belt to switch to the opposite side of the MG unit. Accordingly, two belt tensioners are generally required in such systems, engaging the belt on opposite sides of the MG pulley.
During normal engine driven operation of the generator and other accessories, the loads placed on the belt drive are determined by the power required to drive the accessories, including the MG unit. These accessory drive loads are relatively light and accordingly require belt tensioning only at a moderate low level. Contrariwise, during engine starting, the motor generator unit is required to provide power to rotate the engine as well as the connected accessories. This requires a higher level of belt tensioning to control motion on the slack side of the belt and assure that the belt will not slip. But the higher belt tension may put excessive tensioning loads on the belt and bearings of the system components during normal engine operation, which occupies the vast majority of operating time of the belt drive system. Also, because of packaging requirements it is usually necessary to provide separate tensioner arms with different pivot locations on each side of the MG unit to attain the necessary tensioner arm geometry. An improved accessory drive system, and belt tensioner are accordingly desired.